In recent years, with the aim of speeding up the personal computers or work stations, the operating speed of large scale integrated circuits (LSIs) for use in calculation processing, such as central processing units (CPUs) and digital signal processors (DSPs) continues to increase. The heat generated by such LSIs also increases with the increase in the operating speed (that is, the increase in clock frequency) thereof. The heat generated from the LSI causes the thermal runaway of the LSI itself or affects the peripheral circuits. Therefore, the appropriate cooling down of the heat generators such as the LSI (hereinafter, LSI, collectively) becomes a critical technology. As one example of the technology for cooling down the LSI, an air-cooled method using the cooling fan has been proposed. This method employs, for example, the cooling fan that is disposed opposite to the surface of the LSI so as to blow cool air to the surface of the LSI.
FIG. 1A and FIG. 1B are circuit diagrams illustrating a portion of the fan motor driving device 200r. The output section of the motor driving device comprises an H bridge circuit 206 that is coupled with the motor coil 103 of the driving subject. Regarding the H bridge circuit 206, if it is connected with an external power 106 with the reversed polarity, a high-current may flow therein. To prevent this situation, a reverse connection prevention diode 212 is inserted between the H bridge circuit 206 and the power connector VDD.
In the above-mentioned scenario, it is concerned that the current may flow through the path 112 (comprising the low-voltage side transistor ML1, the motor coil 103, and the body diode DH2 of the high-voltage side transistor) to the external power source 106 based on the phase relationship between the coil current IL and the output voltage VOUT, thereby causing a significant voltage jumping of the output voltage VOUT2 or the source voltage VDD. To solve this problem, the prior technology must insert a Zener diode 108 or smoothing capacitor 110 in parallel connection with the H bridge circuit 206.